While the role of mast cells (MCs) in IgE dependent mediator release leading to respiratory responses such as bronchoconstriction is widely accepted, recent findings have questioned the importance of the MC in such responses. Two groups recently reported that mutant mice virtually devoid of MCs are as susceptible as normal mice to fatal anaphylaxis produced by i.v. antigen challenge after active sensitization (1,2). In contrast, we recently showed that i.v. injection of anti-IgE antibodies produces marked MC degranulation striking changes in pulmonary and cardiovascular functions, and death in naive normal mice, but has none of those effects in naive genetically MC-deficient mice (3). We propose that these apparently discordant findings can be encompassed in a single testable hypothesis: at low levels of IgE (such as in naive mice) MCs are both necessary and sufficient to elicit the pulmonary and cardiovascular effects associated with anaphylaxis, while at higher circulating levels of IgE (such as in mice primed to express active anaphylaxis) MC-independent mechanisms are recruited which permit expression of anaphylaxis in the absence of MCs. Our overall goal is to test this hypothesis under conditions where levels of IgE, and the presence or absence of mast cells, can be controlled and quantitated. We will pursue four specific aims. 1) Define the amounts of exogenous IgE required to achieve similar circulating levels of IgE in MC deficient and normal mice and determine the IgE doses necessary to prime the cells and tissues of mutant and normal mice for optimal mediator release upon stimulation with antigen or anti-IgE; 2) Determine whether MC deficient and normal mice express any significant differences in pulmonary or cardiovascular responsiveness to anaphylactic mediators or their antagonists; 3) Determine whether IgE-mediated degranulation of adoptively transferred MCs of two different phenotypes is sufficient to induce anaphylactic cardiopulmonary changes; 4) Measure cardiopulmonary responses and death associated with anaphylactic stimulation under circumstances in which both the IgE levels and mast cell numbers of the challenged mice are controlled.